1. Field of the Invention
The invention relates to wireless local area network (LAN) system capable of high-rate data transmission through high-rate wireless LAN system compatible with standardized, existing wireless LAN system, and further to a method of driving such wireless LAN system.
2. Description of the Related Art
In a conventional wireless LAN system, there is a problem that since all wireless terminals cannot recognize each other, transmission outputs collide with each other on a wireless channel with the result that a wireless terminal as a receiver cannot normally receive data. In other words, this problem can be set forth as follows. A first wireless terminal starts data transmission while a second wireless terminal is receiving data through a wireless channel, and interferes with normal receipt of data by the second wireless terminal. The first wireless terminal is called a hidden wireless terminal.
In order to solve the problem of a hidden wireless terminal, Japanese Unexamined Patent Publication No. 7-307977 published on Nov. 21, 1995 has suggested a method of communication and a communication apparatus. In the suggested method and apparatus, there is prepared a busy tone channel. A busy tone is transmitted from a particular wireless terminal which transmits and receives data. When other wireless terminals around the particular wireless terminal receive the busy tone through the busy tone channel, the other wireless terminals cease data transmission therefrom. This sort of wireless LAN system is standardized with IEEE 802.11.
FIG. 1 is a timing chart illustrating data to be processed by a conventional wireless LAN system. In a sequence illustrated in FIG. 1, as illustrated in FIG. 1-A, a first wireless terminal as a transmitter transmits RTS frame on a wireless channel through which data communication is to be made. Herein, request to send (RTS) frame means a signal transmitted by a wireless terminal as a transmitter for requesting to make a connection with a wireless terminal as a receiver. In response to RTS frame, a second wireless terminal as a receiver transmits clear to send (CTS) frame as a response signal, as illustrated in FIG. 1-B.
In response to CTS frame, the first wireless terminal as a transmitter transmits a data frame to be transferred, as illustrated in FIG. 1-A. Thereafter, in response to the data frame, the second wireless terminal as a receiver transmits acknowledgement (ACK) frame as a positive response signal, as illustrated in FIG. 1-B. Both RTS and CTS frames contain therein data about a time when ACK frame terminates or falls down.
When another wireless terminal receives RTS frame transmitted from the first wireless terminal, as illustrated in FIG. 1-C, the another wireless terminal ceases data transmission therefrom, from a time when it receives full RTS frame till ACK frame terminates, based on the data contained in RTS frame about a time when ACK frame terminates. On the other hand, if another wireless terminal receives CTS frame transmitted from the second wireless terminal, as illustrated in FIG. 1-D, the another wireless terminal ceases data transmission therefrom, from a time when it receives full CTS frame till ACK frame terminates, based on the data contained in CTS frame about a time when ACK frame terminates.
A modem adopting spectrum scattering process in IEEE 802.11 usually has a data transfer rate in the range of 1 to 2 Mps, and is standardized at a lower data transfer rate than that of wired LAN.
The above-mentioned conventional wireless LAN system has a problem that when a wireless terminal carries out high-rate data transfer in an IEEE 802.11-standardized wireless LAN network area, a lot of hidden wireless terminals which cannot cease data transmission therefrom interfere with normal data receipt of the wireless terminal.
That is, if wireless channel connection is established in an IEEE 802.11-standardized wireless LAN network area through a first modem which is capable of accomplishing high-rate data transfer, a wireless terminal including an IEEE 802.11-standardized second modem accomplishing only low-rate data transfer cannot recognize the wireless channel connection having been established by the first modem, because the first and second modems are different in a data transfer rate. For this reason, there appear a lot of hidden wireless terminals which starts data transmission through a wireless channel while other wireless terminals are making data transmission through the same wireless channel, and as a result, interferes with normal data receipt by the other wireless terminals.
It is an object of the present invention to provide wireless LAN system which can make it possible to establish wireless channel connection and carry out high-rate data transfer through individual modems, and further can solve the above-mentioned problem of a hidden wireless terminal which starts data transmission through a wireless channel while other wireless terminals are making data transmission through the same wireless channel, and interferes with normal data receipt by the other wireless terminals, when high-rate data transfer is to be carried out in an IEEE 802.11-standardized wireless LAN network area.
It is also an object of the present invention to provide a method of driving wireless LAN system by which the problem of a hidden wireless terminal can be solved.
In one aspect, there is provided a wireless LAN system including a wireless terminal which transmits a frame to and receives a frame from other wireless terminals for connecting wireless channel therebetween, and transmits data to and receives data from the other wireless terminals through the wireless channel, wherein the other wireless terminals stop data transmission on receiving the frame from the wire terminal, the wireless terminal being designed to transmit a frame to and receive a frame from the other wireless terminals at a first data transfer rate for connecting wireless channel therebetween, and thereafter transmit data to and receive data from the other wireless terminals at a second date transfer rate greater than the first data transfer rate.
For instance, the wireless terminal may be designed to include (a) a first modem for carrying out modulation and demodulation to accomplish high-rate data transfer, (b) a second modem for carrying out modulation and demodulation to accomplish low-rate data transfer, (c) a data processor for carrying out data transmission and receipt, and also for selecting one of the first and second modems, and (d) a radio frequency processor for transmitting in radio a signal transmitted from one of the first and second modems;, and also for converting a received signal into a signal to be demodulated by one of the first and second modems.
It is preferable that the first modem is a high-rate data transfer modem, and the second modem is a wireless LAN modem.
The data processor may be designed to transmit a modem switching signal, in which case, it is preferable that the wireless LAN system further includes a switch for alternatively switching the first and second modems in accordance with the modem switching signal.
It is preferable that the data processor is designed, when data is to be transmitted, to select the second modem to thereby transmit RTS frame and receive CTS frame from the other wireless terminals for connecting a wireless channel therebetween, and thereafter, select the first modem to thereby transmit a data frame and receive ACK frame to thereby terminate data transmission. It is also preferable that the data processor is designed, when data is to be received, to select the second modem to thereby receive RTS frame from the other wireless terminals and transmit CTS frame for connecting a wireless channel therebetween, and thereafter, select the first modem to thereby receive a data frame and transmit ACK frame to thereby terminate data transmission.
It is preferable that the first modem is designed to transfer data at a rate of at least 4.8 Mbps. It is also preferable that the second modem is designed to transfer data at a rate of at least 2.0 Mbps.
It is preferable that the wireless LAN system further includes a second data processor electrically connected to the data processor for providing transmission data to the data processor and receiving data through the data processor.
In another aspect of the present invention, there is provided a method of driving wireless LAN system wherein a wireless terminal transmits a frame to and receives a frame from other wireless terminals for connecting wireless channel therebetween, and transmits data to and receives data from the other wireless terminals through the wireless channel, the other wireless terminals stopping transmission on receiving the frame from the wire terminal, the method including the steps of (a) transmitting a frame to and receive a frame from the other wireless terminals at a first data transfer rate for connecting wireless channel therebetween, and (b) transmitting data to and receive data from the other wireless terminals at a second data transfer rate greater than the first data transfer rate.
There is further provided a method of driving wireless LAN system including a first modem for carrying out modulation and demodulation to accomplish high-rate data transfer, and a second modem for carrying out modulation and demodulation to accomplish low-rate data transfer, the method including the steps of (a) selecting the second modem for transmitting RTS frame and receiving CTS frame from the other wireless terminals to thereby connect a wireless channel therebetween, and (b) selecting the first modem for transmitting a data frame and receiving ACK frame to thereby terminate data transmission.
There is still further provided a method of driving wireless LAN system including a first modem for carrying out modulation and demodulation to accomplish high-rate data transfer, and a second modem for carrying out modulation and demodulation to accomplish low-rate data transfer, the method including the steps of (a) selecting the second modem for receiving RTS frame from the other wireless terminals and transmitting CTS frame to thereby connect a wireless channel therebetween, and (b) selecting the first modem for receiving a data frame and transmitting ACK frame to thereby terminate data transmission.
In the above-mentioned wireless LAN system in accordance with the present invention, a wireless terminal transmits and receives a frame for establishment of wireless channel connection to thereby start data communication, and other wireless terminals receiving the frame communication cease data transmission. The wireless terminal transmits and receives a frame for establishing wireless channel connection at a first data transfer rate, for instance, at an IEEE 802.11-standardized data transfer rate of 2.0 Mbps, and transmits and receives data at a second data transfer rate greater than the first data transfer rate, for instance, at an IEEE 802.11-standardized data transfer rate of 4.8 Mbps.
Thus, it is possible to solve a problem of a hidden wireless terminal which interferes with other wireless terminals to normally receive data in an IEEE 802.11-standardized wireless LAN network area. That is, for instance, wireless channel connection is established by means of RTS or CTS frame uniformly at an IEEE 802.11-standardized data transfer rate of 2.0 Mbps. In addition, it is also possible to transfer data at a high rate. In other words, when there is employed a modem capable of transferring at a high rate in an IEEE 802.11-standardized wireless LAN network area, a wireless terminal which transfers data at a low rate can surely recognize establishment of wireless channel connection, resulting in that a problem of hidden wireless terminals can be solved.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.